4654
Langmuir 2006, 22, 4654-4659
Measurement of the Azimuthal Anchoring Energy of Liquid Crystals in Contact with Oligo(ethylene glycol)-Terminated Self-Assembled Monolayers Supported on Obliquely Deposited Gold Films Brian H. Clare, Orlando Guzma´n, Juan J. de Pablo, and Nicholas L. Abbott* Department of Chemical and Biological Engineering, UniVersity of WisconsinsMadison, Madison, Wisconsin 53706 ReceiVed December 29, 2005. In Final Form: March 8, 2006 We report measurements of the orientations and azimuthal anchoring energies of the nematic liquid crystal 4-cyano4′-pentylbiphenyl (5CB) on polycrystalline gold films that are deposited from a vapor at an oblique angle of incidence and subsequently decorated with organized monolayers of oligomers of ethylene glycol. Whereas the gold films covered with monolayers presenting tetra(ethylene glycol) (EG4) lead to orientations of 5CB that are perpendicular to the plane of incidence of the gold, monolayers presenting tri(ethylene glycol) (EG3) direct 5CB to orient parallel to the plane of incidence of the gold during deposition of the gold film. We also measure the azimuthal anchoring energy of the 5CB to be smaller on the surfaces presenting EG3 (3.2 ( 0.8 µJ/m2) as compared to EG4 (5.5 ( 0.9 µJ/m2). These measurements, when combined with other results presented in this paper, are consistent with a physical model in which the orientation and anchoring energies of LCs on these surfaces are influenced by both (i) short-range interactions of 5CB with organized oligomers of ethylene glycol at these surfaces and (ii) long-range interactions of 5CB with the nanometer-scale topography of the obliquely deposited films. For surfaces presenting EG3, these shortand long-range interactions oppose each other, leading to small net values of anchoring energies that we predict are dependent on the level of order in the EG3 SAM. These measurements provide insights into the balance of interactions that control the orientational response of LCs to biological species (proteins, viruses, cells) on these surfaces.
Introduction Nematic liquid crystals (LCs) are materials that can exhibit orientational order over distances that are much greater (micrometer) than the sizes of their molecular components (nanometer).1 Near the surface of a structured solid, a LC will typically assume a preferred average orientation due to interactions with the solid.2,3 The orientation of the director of the LC near such an interface, in the absence of an additional external field, is defined as the easy axis, η0. A variety of approaches have been investigated to prepare structured solid surfaces that lead to changes in the orientation of η0, including the mechanical shearing of polymers and the use of microfabricated surfaces with micrometer- and nanometer-scale topography.2,3 An equally important aspect of the phenomenon of the anchoring of LCs at surfaces is the energy of interaction between the substrate and LC. The energy of interaction leading to a particular azimuthal orientation can be characterized by the so-called azimuthal anchoring energy, defined as
τ ) Waz sin 2φ/2
(1)
where τ is the magnitude of torque applied to the LC at the surface (surface anchoring torque) that leads to a departure of the azimuthal orientation of the director of the LC from the easy axis by an angle of φ. Knowledge of Waz permits prediction of the responses of LCs to external perturbations (e.g., electrical or magnetic fields) and also provides fundamental insights into the nature of the interactions between the LCs and surfaces that define the preferred orientations of LCs. In this paper, we report * To whom correspondence should be addressed. Phone: (608) 2655278. Fax: (608) 262-5434. E-mail:
[email protected]. (1) de Gennes, P. G. The Physics of Liquid Crystals, 1st ed.; Oxford University Press: London, 1974. (2) Cognard, J. Mol. Cryst. Liq. Cryst. Suppl. 1982, 78, 1. (3) Jerome, B. Rep. Prog. Phys. 1991, 54, 391.
measurements of the azimuthal anchoring energy of the nematic LC 4-cyano-4′-pentylbiphenyl (5CB) with structured interfaces that present oligomers of ethylene glycol. A number of past studies have reported measurements of the azimuthal anchoring energy of mechanically sheared, polymeric surfaces.4-7 It is now understood that mechanical shearing of polymeric surfaces simultaneously introduces two elements of surface structure that can influence the anchoring energy of a LC: (1) micro- and nanogrooves (topography)8 and (2) preferred alignments of the polymer chains in the near-surface region of the substrate.9,10 Several groups have isolated the contributions that surface topography can make to azimuthal anchoring energies by studying LCs in contact with substrates having feature sizes that can be systematically controlled (e.g. surface gratings11,12 or periodic microrelief structures prepared by the deformation of hard coatings supported on soft polymeric substrates13). Distortion of the director of the LC over the topography of the surface can create an “elastic contribution” to the anchoring energy.8 In addition, the influence of the molecular-level organization of substrate molecules on azimuthal anchoring energies of LCs have been studied independently of topography (4) Sato, Y.; Sato, K.; Uchida, T. Jpn. J. Appl. Phys. 2 1992, 31, L579. (5) Lee, E. S.; Vetter, P.; Miyashita, T.; Uchida, T. Jpn. J. Appl. Phys. 2 1993, 32, L1339. (6) Ban, B. S.; Kim, Y. B. J. Phys. Chem. B 1999, 103, 3869. (7) Oka, S.; Mitsumoto, T.; Kimura, M.; Akahane, T. Phys. ReV. E 2004, 69, 061711. (8) Berreman, D. W. Phys. ReV. Lett. 1972, 28, 1683. (9) Toney, M. F.; Russell, T. P.; Logan, J. A.; Kikuchi, H.; Sands, J. M.; Kumar, S. K. Nature 1995, 374, 709. (10) Stohr, J.; Samant, M. G.; Luning, J.; Callegari, A. C.; Chaudhari, P.; Doyle, J. P.; Lacey, J. A.; Lien, S. A.; Purushothaman, S.; Speidell, J. L. Science 2001, 292, 2299. (11) Newsome, C. J.; O’Neill, M.; Farley, R. J.; Bryan-Brown, G. P. Appl. Phys. Lett. 1998, 72, 2078. (12) Wood, E. L.; Bradberry, G. W.; Cann, P. S.; Sambles, J. R. J. Appl. Phys. 1997, 82, 2483. (13) Belyaev, V.; Misnik, V.; Trofimov, S.; Volynsky, A.; Konovalov, V.; Muravski, A. Appl. Phys. Lett. 2005, 86, 011904.
10.1021/la0535126 CCC: $33.50 © 2006 American Chemical Society Published on Web 04/15/2006
Azimuthal Anchoring Energies of 5CB
Figure 1. Preparation of self-assembled monolayers supported on obliquely deposited gold films. (A) Vapor deposition of gold at the oblique angle of incidence, θi. (B) Immersion of gold films into ethanolic solutions of EG3 or EG4. (C) Schematic illustration of surfaces with nanometer-scale topography that support SAMs.
via the exposure of photoactive polymeric layers to linearly polarized light.14-17 Intermolecular forces, such as van der Waals interactions, define the anchoring energies of LCs on these surfaces. In this paper, we report a methodology that permits measurement of the azimuthal anchoring energy of LCs on selfassembled monolayers (SAMs) of ω-functionalized alkanethiols supported on gold films prepared by the vapor deposition of gold at oblique angles of incidence (θi, measured from the surface normal), as shown in Figure 1A,B. This experimental system permits manipulation of the nanometer-scale topography of the surface18 and the in-plane ordering of organosulfur species that present a wide range of chemical functional groups to the LC (Figure 1C).19 The in-plane ordering of molecules on obliquely deposited gold films is guided by the crystallographic texturing of the underlying gold film (caused by oblique deposition).19,20 The choice of the terminal group of the SAMs also allows one to tune the nature of the molecular-level interaction between substrate and the LC, such as hydrogen-bonding,21 van der Waals,22 or electrical double layer interactions.23 The study we report in this paper is broadly motivated by recent demonstrations that the orientations of LCs can serve to (14) Li, X. T.; Pei, D. H.; Kobayashi, S.; Iimura, Y. Jpn. J. Appl. Phys. 2 1997, 36, L432. (15) Hasegawa, M. Jpn. J. Appl. Phys. 2 2002, 41, L1167. (16) Thieghi, L. T.; Barberi, R.; Bonvent, J. J.; Oliveira, E. A.; Giacometti, J. A.; Balogh, D. T. Phys. ReV. E 2003, 67, 041701. (17) Mitsumoto, T.; Oka, S.; Kimura, M.; Akahane, T. Jpn. J. Appl. Phys. 2005, 44, 4062. (18) Skaife, J. J.; Brake, J. M.; Abbott, N. L. Langmuir 2001, 17, 5448. (19) Follonier, S.; Miller, W. J. W.; Abbott, N. L.; Knoesen, A. Langmuir 2003, 19, 10501. (20) Everitt, D. L.; Miller, W. J. W.; Abbott, N. L.; Zhu, X. D. Phys. ReV. B 2000, 62, R4833. (21) Luk, Y.-Y.; Yang, K.-L.; Cadwell, K.; Abbott, N. L. Surf. Sci. 2004, 570, 43. (22) Gupta, V. K.; Abbott, N. L. Phys. ReV. E 1996, 54, R4540. (23) Shah, R. R.; Abbott, N. L. J. Phys. Chem. B 2001, 105, 4936.
Langmuir, Vol. 22, No. 10, 2006 4655
amplify changes in surface structure brought about by the binding of biological species (e.g., proteins,18,24-26 viruses,27 and cells28) to interfaces. Oligo(ethylene glycol)-terminated SAMs (EGX SAMs) on gold films have formed the basis of some of these approaches,29,30 as they are known to resist (or partially resist) the nonspecific adsorption of proteins.31-36 The study reported here sought to develop a deeper understanding of the behavior of LCs in contact with EGX SAMs, as the molecular-level organization of SAMs and morphology of the supporting gold film are known to influence the sensitivity with which LCs report the presence of bound analytes.18,26 We also note that the study reported in this paper, in which the orientation of LCs depends on the organization of EGX SAMs, is broadly relevant to understanding protein adsorption on these interfaces, as the tendency of EGX SAMs to resist protein adsorption has been proposed to depend on the extent of order present in the monolayer.37-40 Finally, we comment that the methodology described below may also provide approaches to quantitation of LC-based assays for molecular and biomolecular interactions. This paper reports use of the so-called elastic torque-balance method to measure Waz for nematic 5CB in contact with three substrates that were prepared with different nanometer-scale topographies and SAMs.12,41,42 Briefly, two identically treated substrates are used to confine the LC and are arranged such that the easy axes η0 are oriented orthogonally (Figure 2A). The surfaces are spaced apart in a wedge-shaped geometry, such that the thickness of the film of LC varies across the sample (Figure 2B). For sufficiently thin LC films, the bulk elastic torque of the twisted LC competes with the surface anchoring torque, resulting in an equilibrium orientation of the director at ηd (Figure 2C), where the behavior of the twisted LC is governed by the following expression (see ref 42 for derivation)
Waz )
2K22ψ d sin 2φ
(2)
where K22 is the twist elastic constant for the LC, d is the thickness of the film of LC, φ is the angle with which the equilibrium position of the director deviates from the easy axis, and Ψ is the angular twist of the director in the LC. φ and Ψ are measured using an optical method that we have adapted from Fonseca and Galerne,42 where we employ the use of patterned SAMs to (24) Gupta, V. K.; Skaife, J. J.; Dubrovsky, T. B.; Abbott, N. L. Science 1998, 279, 2077. (25) Skaife, J. J.; Abbott, N. L. Langmuir 2000, 16, 3529. (26) Skaife, J. J.; Abbott, N. L. Langmuir 2001, 17, 5595. (27) Tercero Espinoza, L. A.; Schumann, K. R.; Luk, Y.-Y.; Israel, B. A.; Abbott, N. L. Langmuir 2004, 20, 2375. (28) Fang, J.; Ma, W.; Selinger, J. V.; Shashidar, R. Langmuir 2003, 19, 2865. (29) Clare, B. H.; Abbott, N. L. Langmuir 2005, 21, 6451. (30) Luk, Y.-Y.; Tingey, M. L.; Hall, D. J.; Israel, B. A.; Murphy, C. J.; Bertics, P. J.; Abbott, N. L. Langmuir 2003, 19, 1671. (31) Prime, K. L.; Whitesides, G. M. Science 1991, 252, 1164. (32) Pale-Grosdemange, C.; Simon, E. S.; Prime, K. L.; Whitesides, G. M. J. Am. Chem. Soc. 1991, 113, 12. (33) Sigal, G. B.; Bamdad, C.; Barberis, A.; Strominger, J.; Whitesides, G. M. Anal. Chem. 1996, 68, 490. (34) Lahiri, J.; Isaacs, L.; Grzybowski, B.; Carbeck, J. D.; Whitesides, G. M. Langmuir 1999, 15, 7186. (35) Houseman, B. T.; Huh, J. H.; Kron, S. J.; Mrksich, M. Nature Biotechnology 2002, 20, 270. (36) Houseman, B. T.; Gawalt, E. S.; Mrksich, M. Langmuir 2003, 19, 1522. (37) Schwendel, D.; Dahint, R.; Herrwerth, S.; Schloersholz, M.; Eck, W.; Grunze, M. Langmuir 2001, 17, 5717. (38) Harder, P.; Grunze, M.; Dahint, R.; Whitesides, G. M.; Laibinis, P. E. J. Phys. Chem. B 1998, 102, 426. (39) Herrwerth, S.; Eck, W.; Reinhardt, S.; Grunze, M. J. Am. Chem. Soc. 2003, 125, 9359. (40) Vanderah, D. J.; Valincius, G.; Meuse, C. W. Langmuir 2002, 18, 4674. (41) Polossat, E.; Dozov, I. Mol. Cryst. Liq. Cryst. 1996, 282, 223. (42) Fonseca, J. G.; Galerne, Y. Appl. Phys. Lett. 2001, 79, 2910.
4656 Langmuir, Vol. 22, No. 10, 2006
Figure 2. Geometry of LC cell for measurement of azimuthal anchoring energy. (A) Confining surfaces are oriented orthogonally so as to induce a twist distortion in the LC. (B) Wedge-cell construction for control of spacing between surfaces. (C) Diagram depicting the equilibrium position of the director and easy axis near each interface.
determine the angle formed between η0 of the top surface and η0 of the bottom surface (abbreviated as δ). Materials and Methods Materials. All materials were used as received, unless otherwise noted. Fisher’s Finest glass slides were obtained from Fisher Scientific (Pittsburgh, PA). Gold (99.999% purity) was obtained from International Advanced Materials (Spring Valley, NY). Titanium (99.99% purity) was obtained from PureTech (Brewster, NY). Liquid crystal 4-cyano-4′-pentylbiphenyl (5CB) was obtained from EM Industries (New York, NY), sold under the trademark name Licristal (K15). Oligo(ethylene glycol)-terminated thiols were synthesized using previously published methods.32 Ethanol (200 proof) was obtained from Aaper Alcohol (Shelbyville, KY) and purged at least 1 h with argon gas prior to use. Poly(dimethylsiloxane) (PDMS) elastomeric stamps were prepared using Sylgard 184 silicone elastomer kit obtained from Dow Corning (Midland, MI). Preparation of Gold Substrates. Glass slides were first cleaned using a piranha solution as outlined in a prior publication.43 The slides were then positioned within the chamber of an electron beam evaporator such that the incident angle of the flux of metal onto the substrate (depicted in Figure 1A) could be controlled. The incident angles were measured manually using a digital level, with an accuracy of (2°. All metal films were deposited at chamber pressures
